Positive electrode active material for secondary battery, method for preparing the same and lithium secondary battery comprising the same

ABSTRACT

A positive electrode active material for a secondary battery is provided, which includes a lithium composite transition metal oxide including nickel (Ni), cobalt (Co), and manganese (Mn), wherein a particle of the lithium composite transition metal oxide includes a core portion and a resistance portion formed on a surface of the core portion, and is composed of a single particle, wherein the core portion has a layered crystal structure of space group R-3m, and the resistance portion has a cubic rock-salt structure of space group Fm-3m.

TECHNICAL FIELD

The present invention relates to a positive electrode active materialfor a secondary battery, a method of preparing the same, and a lithiumsecondary battery including the positive electrode active material.

BACKGROUND ART

Recently, with the rapid spread of electronic devices using batteries,such as mobile phones, notebook computers, and electric vehicles, demandfor secondary batteries with relatively high capacity as well as smallsize and lightweight has been rapidly increased. Particularly, since alithium secondary battery is lightweight and has high energy density,the lithium secondary battery is in the spotlight as a driving powersource for portable devices. Accordingly, research and developmentefforts for improving the performance of the lithium secondary batteryhave been actively conducted.

In the lithium secondary battery in a state in which an organicelectrolyte solution or a polymer electrolyte solution is filled betweena positive electrode and a negative electrode which are respectivelyformed of active materials capable of intercalating and deintercalatinglithium ions, electrical energy is produced by oxidation and reductionreactions when the lithium ions are intercalated/deintercalatedinto/from the positive electrode and the negative electrode.

Lithium cobalt oxide (LiCoO₂), lithium nickel oxide (LiNiO₂), lithiummanganese oxide (LiMnO₂ or LiMn₂O₄, etc.), or a lithium iron phosphatecompound (LiFePO₄) has been used as a positive electrode active materialof the lithium secondary battery. Among these materials, since thelithium cobalt oxide (LiCoO₂) is advantageous in that its operatingvoltage is high and capacity characteristics are excellent, the lithiumcobalt oxide (LiCoO₂) has been widely used and has been used as apositive electrode active material for high voltage. However, sincethere is a limitation in using a large amount of the LiCoO₂ as a powersource for applications, such as electric vehicles, due to the risingprice and unstable supply of cobalt (Co), there emerges a need todevelop a positive electrode active material capable of replacing theLiCoO₂.

Accordingly, a nickel cobalt manganese-based lithium compositetransition metal oxide (hereinafter, simply referred to as ‘NCM-basedlithium composite transition metal oxide’), in which a portion of cobalt(Co) is substituted with nickel (Ni) and manganese (Mn), has beendeveloped. However, since a conventional NCM-based lithium compositetransition metal oxide is generally in the form of a secondary particlein which primary particles are aggregated, its specific surface area islarge, particle strength is low, and an amount of lithium by-product islarge, and thus, there is a limitation in that an amount of gasgenerated during cell operation is large and stability is poor.Particularly, with respect to a high-Ni NCM-based lithium compositetransition metal oxide in which an amount of nickel (Ni) is increased tosecure high capacity, structural and chemical stabilities are furtherreduced, and it is more difficult to secure thermal stability.

PRIOR ART DOCUMENT Patent Document

(Patent Document 1) Korean Patent Application Laid-open Publication No.2016-0074236

DISCLOSURE OF THE INVENTION Technical Problem

An aspect of the present invention provides an NCM-based lithiumcomposite transition metal oxide positive electrode active materialhaving improved stability. Specifically, the present invention aims atproviding an NCM-based lithium composite transition metal oxide positiveelectrode active material in which particle breakage during rolling issuppressed by decreasing a specific surface area and improving particlestrength, and an increase in resistance is suppressed and a sidereaction with an electrolyte solution is reduced by decreasing an amountof lithium by-product. Also, the present invention aims at providing anNCM-based lithium composite transition metal oxide positive electrodeactive material in which an amount of gas generated during celloperation may be reduced and thermal stability is secured.

Technical Solution

According to an aspect of the present invention, there is provided amethod of preparing a positive electrode active material for a secondarybattery which includes: preparing a positive electrode active materialprecursor including nickel (Ni), cobalt (Co), and manganese (Mn) inwhich an amount of the nickel (Ni) in a total amount of metals is lessthan 60 mol%; mixing the positive electrode active material precursorand a lithium raw material, and performing primary sintering on themixture at a sintering temperature of 980° C. or more to form a primarysintered product; and performing secondary sintering on the primarysintered product at a sintering temperature of 900° C. or less to form alithium composite transition metal oxide, wherein a particle of thelithium composite transition metal oxide is composed of a singleparticle and includes a core portion having a layered crystal structureof space group R-3m; and a resistance portion which is formed on asurface of the core portion and has a cubic rock-salt structure of spacegroup Fm-3m.

According to another aspect of the present invention, there is provideda method of preparing a positive electrode active material for asecondary battery which includes: preparing a positive electrode activematerial precursor including nickel (Ni), cobalt (Co), and manganese(Mn) in which an amount of the nickel (Ni) in a total amount of metalsis 60 mol % or more; mixing the positive electrode active materialprecursor and a lithium raw material, and performing primary sinteringon the mixture at a sintering temperature of 850° C. or more to form aprimary sintered product; and performing secondary sintering on theprimary sintered product at a sintering temperature of 800° C. or lessto form a lithium composite transition metal oxide, wherein a particleof the lithium composite transition metal oxide is composed of singleparticles and includes a core portion having a layered crystal structureof space group R-3m; and a resistance portion which is formed on asurface of the core portion and has a cubic rock-salt structure of spacegroup Fm-3m.

According to another aspect of the present invention, there is provideda positive electrode active material for a secondary battery whichincludes a lithium composite transition metal oxide including nickel(Ni), cobalt (Co), and manganese (Mn), wherein a particle of the lithiumcomposite transition metal oxide includes a core portion and aresistance portion formed on a surface of the core portion and iscomposed of a single particle, wherein the core portion has a layeredcrystal structure of space group R-3m, and the resistance portion has acubic rock-salt structure of space group Fm-3m.

According to another aspect of the present invention, there is provideda positive electrode and a lithium secondary battery which include thepositive electrode active material.

Advantageous Effects

According to the present invention, a side reaction with an electrolytesolution may be reduced by decreasing a specific surface area of anNCM-based positive electrode active material, improving particlestrength, and decreasing an amount of lithium by-product. Thus, withrespect to a lithium secondary battery using the NCM-based positiveelectrode active material of the present invention, an amount of gasgenerated during cell operation may be reduced, an increase inresistance may be suppressed, and thermal stability may be secured.Since the NCM-based positive electrode active material of the presentinvention may secure excellent stability, it may be used in ahigh-voltage lithium secondary battery.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings attached to the specification illustratepreferred examples of the present invention by example, and serve toenable technical concepts of the present invention to be furtherunderstood together with detailed description of the invention givenbelow, and therefore the present invention should not be interpretedonly with matters in such drawings.

FIG. 1 is a magnified scanning electron microscope (SEM) image of apositive electrode active material prepared in Example 1;

FIG. 2 is a magnified scanning electron microscope (SEM) image of apositive electrode active material prepared in Example 2;

FIG. 3 is a magnified scanning electron microscope (SEM) image of apositive electrode active material prepared in Example 3;

FIG. 4 is a magnified scanning electron microscope (SEM) image of apositive electrode active material prepared in Example 4;

FIG. 5 is a magnified scanning electron microscope (SEM) image of apositive electrode active material prepared in Example 5;

FIG. 6 is a magnified scanning electron microscope (SEM) image of apositive electrode active material prepared in Example 6;

FIG. 7 is a magnified scanning electron microscope (SEM) image of apositive electrode active material prepared in Comparative Example 1;

FIG. 8 is a magnified scanning electron microscope (SEM) image of apositive electrode active material prepared in Comparative Example 2;

FIG. 9 is a magnified scanning electron microscope (SEM) image of apositive electrode active material prepared in Comparative Example 3;

FIG. 10 is a magnified scanning electron microscope (SEM) image of apositive electrode active material prepared in Comparative Example 4;

FIG. 11 is a transmission electron microscope (TEM) image of thepositive electrode active material prepared in Example 1;

FIG. 12 is a transmission electron microscope (TEM) image of thepositive electrode active material prepared in Comparative Example 1;

FIG. 13 is a transmission electron microscope (TEM) image of thepositive electrode active material prepared in Example 4; and

FIG. 14 is a transmission electron microscope (TEM) image of thepositive electrode active material prepared in Comparative Example 3.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail toallow for a clearer understanding of the present invention. In thiscase, it will be understood that words or terms used in thespecification and claims shall not be interpreted as the meaning definedin commonly used dictionaries, and it will be further understood thatthe words or terms should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thetechnical idea of the invention, based on the principle that an inventormay properly define the meaning of the words or terms to best explainthe invention.

Method of Preparing Positive Electrode Active Material

A positive electrode active material of the present invention isprepared by including the steps of: preparing a positive electrodeactive material precursor including nickel (Ni), cobalt (Co), andmanganese (Mn) in which an amount of the nickel (Ni) in a total amountof metals is less than 60 mol %; mixing the positive electrode activematerial precursor and a lithium raw material, and performing primarysintering on the mixture at a sintering temperature of 980° C. or moreto form a primary sintered product; and performing secondary sinteringon the primary sintered product at a sintering temperature of 900° C. orless to form a lithium composite transition metal oxide, wherein aparticle of the lithium composite transition metal oxide is composed ofa single particle and includes a core portion having a layered crystalstructure of space group R-3m; and a resistance portion which is formedon a surface of the core portion and has a cubic rock-salt structure ofspace group Fm-3m.

Also, the positive electrode active material of the present invention isprepared by including the steps of: preparing a positive electrodeactive material precursor including nickel (Ni), cobalt (Co), andmanganese (Mn) in which an amount of the nickel (Ni) in a total amountof metals is 60 mol % or more; mixing the positive electrode activematerial precursor and a lithium raw material, and performing primarysintering on the mixture at a sintering temperature of 850° C. or moreto form a primary sintered product; and performing secondary sinteringon the primary sintered product at a sintering temperature of 800° C. orless to form a lithium composite transition metal oxide, wherein aparticle of the lithium composite transition metal oxide is composed ofsingle particles and includes a core portion having a layered crystalstructure of space group R-3m; and a resistance portion which is formedon a surface of the core portion and has a cubic rock-salt structure ofspace group Fm-3m.

A method of preparing the positive electrode active material will bedescribed in detail for each step.

First, a positive electrode active material precursor including nickel(Ni), cobalt (Co), and manganese (Mn) is prepared.

The positive electrode active material precursor may be used bypurchasing a commercially available positive electrode active materialprecursor, or may be prepared according to a method of preparing apositive electrode active material precursor which is well known in theart.

For example, the precursor may be prepared by a co-precipitationreaction by adding an ammonium cation-containing complexing agent and abasic compound to a transition metal solution including anickel-containing raw material, a cobalt-containing raw material, and amanganese-containing raw material.

The nickel-containing raw material, for example, may includenickel-containing acetic acid salts, nitrates, sulfates, halides,sulfides, hydroxides, oxides, or oxyhydroxides, and may specificallyinclude Ni(OH)₂, NiO, NiOOH, NiCO₃.2Ni (OH)₂.4H₂O, NiC₂O₂.2H₂O, Ni(NO₃)₂.6H₂O, NiSO₄, NiSO₄.6H₂O, a fatty acid nickel salt, a nickelhalide, or a combination thereof, but the present invention is notlimited thereto.

The cobalt-containing raw material may include cobalt-containing aceticacid salts, nitrates, sulfates, halides, sulfides, hydroxides, oxides,or oxyhydroxides, and may specifically include Co(OH)₂, CoOOH,Co(OCOCH₃)₂.4H₂O, Co(NO₃)₂.6H₂O, Co(SO₄)₂, Co(SO₄)₂.7H₂O, or acombination thereof, but the present invention is not limited thereto.

The manganese-containing raw material, for example, may includemanganese-containing acetic acid salts, nitrates, sulfates, halides,sulfides, hydroxides, oxides, oxyhydroxides, or a combination thereof,and may specifically include a manganese oxide such as Mn₂O₃, MnO₂, andMn₃O₄; a manganese salt such as MnCO₃, Mn(NO₃)₂, MnSO₄, manganeseacetate, manganese dicarboxylate, manganese citrate, and a fatty acidmanganese salt; a manganese oxyhydroxide, manganese chloride, or acombination thereof, but the present invention is not limited thereto.

The transition metal solution may be prepared by adding thenickel-containing raw material, the cobalt-containing raw material, andthe manganese-containing raw material to a solvent, specifically water,or a mixture of water and an organic solvent (e.g., alcohol etc.) whichmay be uniformly mixed with the water, or may be prepared by mixing anaqueous solution of the nickel-containing raw material, an aqueoussolution of the cobalt-containing raw material, and themanganese-containing raw material.

The ammonium cation-containing complexing agent, for example, mayinclude NH₄OH, (NH₄)₂SO₄, NH₄NO₃, NH₄Cl, CH₃COONH₄, NH₄CO₃, or acombination thereof, but the present invention is not limited thereto.The ammonium cation-containing complexing agent may be used in the formof an aqueous solution, and, in this case, water or a mixture of waterand an organic solvent (specifically, alcohol etc.), which may beuniformly mixed with the water, may be used as a solvent.

The basic compound may include a hydroxide of alkali metal or alkalineearth metal, such as NaOH, KOH, or Ca(OH)₂, a hydrate thereof, or acombination thereof. The basic compound may also be used in the form ofan aqueous solution, and, in this case, water or a mixture of water andan organic solvent (specifically, alcohol etc.), which may be uniformlymixed with the water, may be used as a solvent.

The basic compound is added to adjust a pH of a reaction solution,wherein the basic compound may be added in an amount such that the pH ofthe metal solution is 11 to 13.

The co-precipitation reaction may be performed in a temperature range of40° C. to 70° C. in an inert atmosphere such as nitrogen or argon.

Particles of a nickel-cobalt-manganese hydroxide are formed by theabove-described process, and are precipitated in the reaction solution.Concentrations of the nickel-containing raw material, thecobalt-containing raw material, and the manganese-containing rawmaterial may be adjusted to prepare a precursor in which the amount ofthe nickel (Ni) in the total amount of metals is 60 mol % or more or toprepare a precursor in which the amount of the nickel (Ni) in the totalamount of metals is less than 60 mol %. The precipitatednickel-cobalt-manganese hydroxide particles may be separated accordingto a conventional method and dried to prepare a nickel-cobalt-manganeseprecursor. The precursor may be a secondary particle which is formed byaggregation of primary particles.

Next, the positive electrode active material precursor and a lithium rawmaterial are mixed and primarily sintered at an over-sinteringtemperature of a specific temperature or more to form a primary sinteredproduct. In a case in which the amount of the nickel (Ni) in the totalamount of metals of the positive electrode active material precursor isless than 60 mol %, the mixture is primarily sintered at a sinteringtemperature of 980° C. or more to form a primary sintered product. Also,in a case in which the amount of the nickel (Ni) in the total amount ofmetals of the positive electrode active material precursor is 60 mol %or more, the mixture is primarily sintered at a sintering temperature of850° C. or more to form a primary sintered product.

As the lithium raw material, lithium-containing sulfates, nitrates,acetic acid salts, carbonates, oxalates, citrates, halides, hydroxides,or oxyhydroxides may be used, and these materials are not particularlylimited as long as they may be dissolved in water. Specifically, thelithium raw material may include Li₂CO₃, LiNO₃, LiNO₂, LiOH, LiOH.H₂O,LiH, LiF, LiCl, LiBr, LiI, CH₃COOLi, Li₂O, Li₂SO₄, CH₃COOLi, orLi₃C₆H₅O₇, and any one thereof or a mixture of two or more thereof maybe used.

In the case that the amount of the nickel (Ni) is less than 60 mol %,the primary sintering temperature may be 980° C. or more, preferably990° C. to 1,050° C., and more preferably 1,000° C. to 1,030° C. Theprimary sintering may be performed for 5 hours to 20 hours, preferably 5hours to 15 hours, and more preferably 5 hours to 12 hours. The primarysintering may be performed in an oxygen atmosphere or an air atmosphere,and may more preferably be performed in an oxygen atmosphere.

In the case that the amount of the nickel (Ni) is 60 mol % or more, theprimary sintering temperature may be 850° C. or more, preferably 850° C.to 1,000° C., and more preferably 900° C. to 950° C. The primarysintering may be performed for 5 hours to 20 hours, preferably 5 hoursto 15 hours, and more preferably 5 hours to 12 hours. The primarysintering may be performed in an oxygen atmosphere or an air atmosphere,and may more preferably be performed in an oxygen atmosphere.

Since the positive electrode active material precursor, which has beenin the form of a secondary particle in which primary particles areaggregated, becomes a single particle by the primary sintering, aprimary sintered product of the NCM-based lithium composite transitionmetal oxide, as a single particle, may be formed.

A step of milling the primary sintered product after the primarysintering and before the secondary sintering may be further included.Specifically, the primary sintered product may be milled using a jetmill. An Fm-3m resistance portion, which is formed during the secondarysintering, may be formed on a surface of the final particle byperforming the step of milling the primary sintered product. If millingis performed after performing the secondary sintering without themilling of the primary sintered product, the Fm-3m resistance portionmay not be uniformly formed on the surface of the final particle.

Next, the primary sintered product is secondarily sintered at asintering temperature, which is lower than the primary sinteringtemperature by a predetermined level, to form a lithium compositetransition metal oxide. In a case in which the amount of the nickel (Ni)is less than 60 mol %, the secondary sintering is performed at asintering temperature of 900° C. or less to form a lithium compositetransition metal oxide. Also, in a case in which the amount of thenickel (Ni) is 60 mol % or more, the secondary sintering is performed ata sintering temperature of 800° C. or less to form a lithium compositetransition metal oxide.

In the case that the amount of the nickel (Ni) is less than 60 mol %,the secondary sintering temperature may be 900° C. or less, preferably600° C. to 900° C., and more preferably 700° C. to 900° C. The secondarysintering may be performed for 5 hours to 20 hours, preferably 5 hoursto 15 hours, and more preferably 5 hours to 10 hours. The secondarysintering may be performed in an oxygen atmosphere or an air atmosphere,and may more preferably be performed in an oxygen atmosphere.

In the case that the amount of the nickel (Ni) is 60 mol % or more, thesecondary sintering temperature may be 800° C. or less, preferably 500°C. to 800° C., and more preferably 600° C. to 800° C. The secondarysintering may be performed for 5 hours to 20 hours, preferably 5 hoursto 15 hours, and more preferably 5 hours to 10 hours. The secondarysintering may be performed in an oxygen atmosphere or an air atmosphere,and may more preferably be performed in an oxygen atmosphere.

A resistance portion having a cubic rock-salt structure of space groupFm-3m may be formed on the surface of the particle by the secondarysintering.

The lithium composite transition metal oxide particle thus prepared isformed of a single particle and includes a core portion having a layeredcrystal structure of space group R-3m; and a resistance portion which isformed on the surface of the core portion and has a cubic rock-saltstructure of space group Fm-3m. Also, the lithium composite transitionmetal oxide particle prepared by the primary over-sintering and thesecondary sintering as described above may be composed of a primaryparticle having an average particle diameter (D₅₀) of 1 μm to 20 μm. Thelithium composite transition metal oxide particle may be preferablycomposed of a primary particle having an average particle diameter (D₅₀)of 2 μm to 15 μm and may be more preferably composed of a primaryparticle having an average particle diameter (D₅₀) of 3 μm to 10 μm.Furthermore, the core portion of the lithium composite transition metaloxide particle prepared by the primary over-sintering and the secondarysintering as described above may have a crystallite size of 180 nm ormore.

The core portion may preferably have a crystallite size of 180 nm to 400nm, and may more preferably have a crystallite size of 180 nm to 300 nm.

Positive Electrode Active Material

Next, the positive electrode active material of the present inventionprepared as described above will be described.

The positive electrode active material for a secondary battery of thepresent invention is a lithium composite transition metal oxideincluding nickel (Ni), cobalt (Co), and manganese (Mn), wherein aparticle of the lithium composite transition metal oxide includes a coreportion and a resistance portion formed on a surface of the core portionand is composed of a single particle, wherein the core portion has alayered crystal structure of space group R-3m, and the resistanceportion has a cubic rock-salt structure of space group Fm-3m.

The positive electrode active material of the present invention is anNCM-based lithium composite transition metal oxide including nickel(Ni), cobalt (Co), and manganese (Mn). The lithium composite transitionmetal oxide may be a high-Ni NCM-based lithium composite transitionmetal oxide in which an amount of the nickel (Ni) in a total amount ofmetals excluding lithium (Li) is 60 mol % or more, and may be a low-NiNCM-based lithium composite transition metal oxide in which the amountof the nickel (Ni) is less than 60 mol %. With respect to the high-NiNCM-based lithium composite transition metal oxide, the amount of thenickel (Ni) may be preferably 65 mol % or more, and may be morepreferably 80 mol % or more. Since the amount of the nickel (Ni) in thetotal amount of the metals excluding lithium (Li) of the lithiumcomposite transition metal oxide satisfies 60 mol % or more, highercapacity may be secured.

Specifically, the positive electrode active material according to anembodiment of the present invention may be an NCM-based lithiumcomposite transition metal oxide represented by Formula 1 below.

Li_(p)Ni_(1−(x1+y1+z1))CO_(x1)Mn_(y1)M^(a) _(z1)O_(2+δ)   [Formula 1]

In Formula 1, M^(a) is at least one element selected from the groupconsisting of aluminum (Al), zirconium (Zr), titanium (Ti), magnesium(Mg), tantalum (Ta), niobium (Nb), molybdenum (Mo), chromium (Cr),barium (Ba), strontium (Sr), and calcium (Ca), and 1≤p≤1.3, 0<x1≤0.5,0<y1≤0.5, 0≤z1≤0.1, and −0.1≤δ≤1.

In the lithium composite transition metal oxide of Formula 1, Li may beincluded in an amount corresponding to p, that is, 1≤p≤1.3. When p isless than 1, capacity may be reduced, and, when p is greater than 1.3,milling is difficult due to an increase in strength of the sinteredpositive electrode active material and there may be an increase inamount of gas generated due to an increase in Li by-product. The Li maymore preferably be included in an amount satisfying 1.0≤p≤1.1, inconsideration of balance between a capacity characteristics improvementeffect of the positive electrode active material and sinterabilityduring the preparation of the active material due to the control of theamount of the Li.

In the lithium composite transition metal oxide of Formula 1, Ni may beincluded in an amount corresponding to 1−(x1+y1+z1), for example,0<1−(x1+y1+z1)≤0.99. If the amount of the Ni in the lithium compositetransition metal oxide of Formula 1 is 0.6 or more, since the amount ofNi, which is sufficient to contribute to charge and discharge, issecured, higher capacity may be achieved. The Ni may more preferably beincluded in an amount satisfying 0.5≤1−(x1+y1+z1)≤0.9.

In the lithium composite transition metal oxide of Formula 1, Co may beincluded in an amount corresponding to x1, that is, 0<x1≤0.5. In a casein which the amount of the Co in the lithium composite transition metaloxide of Formula 1 is greater than 0.5, there is a concern that cost mayincrease. The Co may specifically be included in an amount satisfying0.2≤x1≤0.4 in consideration of a significant capacity characteristicsimprovement effect due to the inclusion of the Co.

In the lithium composite transition metal oxide of Formula 1, Mn may beincluded in an amount corresponding to y1, that is, 0<y1≤0.5. Mn mayimprove stability of the positive electrode active material, and, as aresult, may improve stability of the battery. The Mn may specifically beincluded in an amount satisfying 0.05≤y1≤0.2.

In the lithium composite transition metal oxide of Formula 1, M^(a) maybe a doping element included in a crystal structure of the lithiumcomposite transition metal oxide, wherein the M^(a) may be included inan amount corresponding to z1, that is, 0≤z1≤0.1.

The lithium composite transition metal oxide particle of the presentinvention is not in the form of an aggregated secondary particle, but iscomposed of a single particle, that is, a primary particle. In thepresent invention, the expression ‘primary particle’ denotes a primarystructure of the single particle, and the expression ‘secondaryparticle’ denotes an aggregate in which primary particles are aggregatedby physical or chemical bonding between the primary particles without anintentional aggregation or assembly process of the primary particlesconstituting the secondary particle, that is, a secondary structure.

The lithium composite transition metal oxide particle may be composed ofa primary particle having an average particle diameter (D₅₀) of 1 μm to20 μm. The lithium composite transition metal oxide particle maypreferably be composed of a primary particle having an average particlediameter (D₅₀) of 2 μm to 15 μm and may more preferably be composed of aprimary particle having an average particle diameter (D₅₀) of 3 μm to 10μm. Since the core portion is composed of the primary particle having anaverage particle diameter within the above range, that is, the singleparticle, particle strength may be increased to suppress particlebreakage during rolling, improve rolling density, and reduce the amountof gas generated by a side reaction with an electrolyte solution due todecreases in specific surface area and lithium by-product.

In the present invention, the average particle diameter (D₅₀) may bedefined as a particle diameter at a cumulative volume of 50% in aparticle size distribution curve. The average particle diameter (D₅₀),for example, may be measured by using a laser diffraction method. Forexample, in a method of measuring the average particle diameter (D₅₀) ofthe positive electrode active material, after the particles of thepositive electrode active material are dispersed in a dispersion medium,the dispersion medium is introduced into a commercial laser diffractionparticle size measurement instrument (e.g., Microtrac MT 3000) andirradiated with ultrasonic waves having a frequency of about 28 kHz andan output of 40 W, and the average particle diameter (D₅₀) at thecumulative volume of 50% may then be calculated by the measurementinstrument.

Also, the lithium composite transition metal oxide particle includes acore portion and a resistance portion formed on the surface of the coreportion, wherein the core portion of the lithium composite transitionmetal oxide particle has a layered crystal structure of space groupR-3m, and the resistance portion has a cubic rock-salt structure ofspace group Fm-3m.

In the present invention, the crystal structures of the core portion andthe resistance portion may be identified by high-resolution transmissionelectron microscope (HR-TEM) analysis.

The expression “layered crystal structure of space group R-3m” denotes astructure in which planes of atoms strongly bonded by covalent bonds orthe like and densely arranged are overlapped in parallel by a weakbinding force such as a van der Waals force. With respect to the lithiumcomposite transition metal oxide having a layered crystal structure ofspace group R-3m, intercalation and deintercalation of lithium ions arepossible because the lithium ions, transition metal ions, and oxygenions are densely arranged, specifically, a metal oxide layer composed oftransition metal and oxygen and an oxygen octahedral layer surroundinglithium are alternatingly arranged with each other, and a Coulombrepulsive force acts between the metal oxide layers, and ionicconductivity is high because the lithium ions diffuse along atwo-dimensional plane. Thus, with respect to the positive electrodeactive material having a layered crystal structure of space group R-3m,since the lithium ions may quickly and smoothly move in the particle tofacilitate the intercalation and deintercalation of the lithium ions,initial internal resistance of the battery may be reduced, and thus,discharge capacity and life characteristics may be further improvedwithout worrying about the degradation of rate capability and initialcapacity characteristics.

The expression “cubic rock-salt structure of space group Fm-3m” denotesa face-centered cubic structure in which a metal atom is coordinated bysurrounding six oxygen atoms arranged in an octahedral form. A compoundhaving the cubic rock-salt structure of space group Fm-3m has highstructural stability, particularly, high structural stability at hightemperature.

The core portion may have a crystallite size of 180 nm or more. The coreportion may preferably have a crystallite size of 180 nm to 400 nm, forexample, 180 nm to 300 nm. Since the core portion satisfying the abovecrystallite size according to an embodiment of the present invention isformed, the positive electrode active material may suppress the particlebreakage caused by rolling and the life characteristics and stabilitymay be improved.

In the present invention, the expression ‘particle’ denotes a granulewith a size of a few tens of microns, and, when the particle ismagnified and observed, the particle may be identified as ‘grain’ whichhas a crystal form with a size of a few hundreds of nanometers to a fewmicrons. When the grain is further magnified, it is possible to identifya separated region having a form in which atoms form a lattice structurein a predetermined direction, wherein the region is referred to as a‘crystallite’, and a size of the particle observed by X-ray diffraction(XRD) is defined as a size of the crystallite. With respect to a methodof measuring the crystallite size, the crystallite size may bedetermined by peak broadening of XRD data and may be quantitativelycalculated from the Scherrer equation.

The resistance portion formed on the surface of the core portion has acubic rock-salt structure of space group Fm-3m. The resistance portionmay be formed on a part or entirety of the surface of the core portion.In a case in which the resistance portion is formed on the part of thesurface of the core portion, the resistance portion may be formed in anisland shape.

Since the side reaction with the electrolyte solution is reduced byforming the resistance portion having a cubic rock-salt structure ofspace group Fm-3m on the surface of the core portion, the amount of thegas generated during cell operation may be reduced, the increase inresistance may be suppressed, and the thermal stability may be improved.

Since the positive electrode active material according to the embodimentof the present invention is composed of a single particle and includesthe core portion having a layered crystal structure of space group R-3mand the resistance portion which is formed on the surface of the coreportion and has a cubic rock-salt structure of space group Fm-3m, thespecific surface area of the NCM-based positive electrode activematerial may be reduced, the particle strength may be improved, and theside with the electrolyte solution may be reduced. Thus, with respect toa lithium secondary battery using the NCM-based positive electrodeactive material of the present invention, the amount of the gasgenerated during cell operation may be reduced, the increase inresistance may be suppressed, and the thermal stability may be improved.

In a case in which the positive electrode active material according tothe embodiment of the present invention having the core portion and theresistance portion is a low-Ni NCM-based positive electrode activematerial in which the amount of the nickel (Ni) in the total amount ofmetals excluding lithium (Li) is less than 60 mol %, the positiveelectrode active material may be formed by performing primaryover-sintering at a sintering temperature of 980° C. or more andperforming secondary sintering at a sintering temperature of 900° C. orless.

Also, with respect to a high-Ni NCM-based positive electrode activematerial in which the amount of the nickel (Ni) in the total amount ofmetals excluding lithium (Li) is 60 mol % or more, the positiveelectrode active material may be formed by performing primaryover-sintering at a sintering temperature of 850° C. or more andperforming secondary sintering at a sintering temperature of 800° C. orless.

With respect to the low-Ni NCM-based positive electrode active materialin which the amount of the nickel (Ni) in the total amount of metalsexcluding lithium (Li) is less than 60 mol %, when the positiveelectrode active material according to the embodiment of the presentinvention is thermally analyzed by differential scanning calorimetry(DSC), a main peak with a maximum heat flow may be measured at 280° C.or more. The main peak may preferably be measured at 280° C. to 300° C.and may more preferably be measured at 283° C. to 300° C. Also, themaximum heat flow may be 8 W/g or less, preferably 7 W/g or less, andmore preferably 6 W/g or less.

Furthermore, with respect to the high-Ni NCM-based positive electrodeactive material in which the amount of the nickel (Ni) in the totalamount of metals excluding lithium (Li) is 60 mol % or more, a main peakwith a maximum heat flow may be measured at 210° C. or more. The mainpeak may preferably be measured at 210° C. to 250° C. and may morepreferably be measured at 213° C. to 250° C. Also, the maximum heat flowmay be 15 W/g or less, preferably 13 W/g or less, and more preferably 12W/g or less.

Positive Electrode and Secondary Battery

According to another embodiment of the present invention, provided are apositive electrode for a secondary battery and a lithium secondarybattery which include the above positive electrode active material.

Specifically, the positive electrode includes a positive electrodecollector and a positive electrode active material layer which isdisposed on the positive electrode collector and includes the positiveelectrode active material.

In the positive electrode, the positive electrode collector is notparticularly limited as long as it has conductivity without causingadverse chemical changes in the battery, and, for example, stainlesssteel, aluminum, nickel, titanium, fired carbon, or aluminum orstainless steel that is surface-treated with one of carbon, nickel,titanium, silver, or the like may be used. Also, the positive electrodecollector may typically have a thickness of 3 μm to 500 μm, andmicroscopic irregularities may be formed on the surface of the collectorto improve the adhesion of the positive electrode active material. Thepositive electrode collector, for example, may be used in various shapessuch as that of a film, a sheet, a foil, a net, a porous body, a foambody, a non-woven fabric body, and the like.

Also, the positive electrode active material layer may include aconductive agent and a binder in addition to the above-describedpositive electrode active material.

In this case, the conductive agent is used to provide conductivity tothe electrode, wherein any conductive agent may be used withoutparticular limitation as long as it has suitable electron conductivitywithout causing adverse chemical changes in the battery. Specificexamples of the conductive agent may be graphite such as naturalgraphite or artificial graphite; carbon based materials such as carbonblack, acetylene black, Ketjen black, channel black, furnace black, lampblack, thermal black, and carbon fibers; powder or fibers of metal suchas copper, nickel, aluminum, and silver; conductive whiskers such aszinc oxide whiskers and potassium titanate whiskers; conductive metaloxides such as titanium oxide; or conductive polymers such aspolyphenylene derivatives, and any one thereof or a mixture of two ormore thereof may be used. The conductive agent may be typically includedin an amount of 1 wt % to 30 wt % based on a total weight of thepositive electrode active material layer.

Furthermore, the binder improves the adhesion between the positiveelectrode active material particles and the adhesion between thepositive electrode active material and the current collector. Specificexamples of the binder may be polyvinylidene fluoride (PVDF),polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP),polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC),starch, hydroxypropyl cellulose, regenerated cellulose,polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene,an ethylene-propylene-diene monomer (EPDM), a sulfonated EPDM, astyrene-butadiene rubber (SBR), a fluorine rubber, or various copolymersthereof, and any one thereof or a mixture of two or more thereof may beused. The binder may be included in an amount of 1 wt % to 30 wt % basedon the total weight of the positive electrode active material layer.

The positive electrode may be prepared according to a typical method ofpreparing a positive electrode except that the above-described positiveelectrode active material is used. Specifically, a composition forforming a positive electrode active material layer, which includes theabove-described positive electrode active material as well asselectively the binder and the conductive agent, is coated on thepositive electrode collector, and the positive electrode may then beprepared by drying and rolling the coated positive electrode collector.In this case, types and amounts of the positive electrode activematerial, the binder, and the conductive are the same as thosepreviously described.

The solvent may be a solvent normally used in the art. The solvent mayinclude dimethyl sulfoxide (DMSO), isopropyl alcohol,N-methylpyrrolidone (NMP), acetone, or water, and any one thereof or amixture of two or more thereof may be used. An amount of the solventused may be sufficient if the solvent may dissolve or disperse thepositive electrode active material, the conductive agent, and the binderin consideration of a coating thickness of a slurry and manufacturingyield, and may allow to have a viscosity that may provide excellentthickness uniformity during the subsequent coating for the preparationof the positive electrode.

Also, as another method, the positive electrode may be prepared bycasting the composition for forming a positive electrode active materiallayer on a separate support and then laminating a film separated fromthe support on the positive electrode collector.

According to another embodiment of the present invention, anelectrochemical device including the positive electrode is provided. Theelectrochemical device may specifically be a battery or a capacitor,and, for example, may be a lithium secondary battery.

The lithium secondary battery specifically includes a positiveelectrode, a negative electrode disposed to face the positive electrode,a separator disposed between the positive electrode and the negativeelectrode, and an electrolyte, wherein the positive electrode is asdescribed above. Also, the lithium secondary battery may furtherselectively include a battery container accommodating an electrodeassembly of the positive electrode, the negative electrode, and theseparator, and a sealing member sealing the battery container.

In the lithium secondary battery, the negative electrode includes anegative electrode collector and a negative electrode active materiallayer disposed on the negative electrode collector.

The negative electrode collector is not particularly limited as long asit has high conductivity without causing adverse chemical changes in thebattery, and, for example, copper, stainless steel, aluminum, nickel,titanium, fired carbon, copper or stainless steel that issurface-treated with one of carbon, nickel, titanium, silver, or thelike, and an aluminum-cadmium alloy may be used. Also, the negativeelectrode collector may typically have a thickness of 3 μm to 500 μm,and, similar to the positive electrode collector, microscopicirregularities may be formed on the surface of the collector to improvethe adhesion of a negative electrode active material. The negativeelectrode collector, for example, may be used in various shapes such asthat of a film, a sheet, a foil, a net, a porous body, a foam body, anon-woven fabric body, and the like.

The negative electrode active material layer selectively includes abinder and a conductive agent in addition to the negative electrodeactive material. The negative electrode active material layer may beprepared by coating a composition for forming a negative electrode inthe form of a slurry, which includes selectively the binder and theconductive agent as well as the negative electrode active material, onthe negative electrode collector and drying the coated negativeelectrode collector, or may be prepared by casting the composition forforming a negative electrode on a separate support and then laminating afilm separated from the support on the negative electrode collector.

A compound capable of reversibly intercalating and deintercalatinglithium may be used as the negative electrode active material. Specificexamples of the negative electrode active material may be a carbonaceousmaterial such as artificial graphite, natural graphite, graphitizedcarbon fibers, and amorphous carbon; a metallic compound alloyable withlithium such as silicon (Si), aluminum (Al), tin (Sn), lead (Pb), zinc(Zn), bismuth (Bi), indium (In), magnesium (Mg), gallium (Ga), cadmium(Cd), a Si alloy, a Sn alloy, or an Al alloy; a metal oxide which may bedoped and undoped with lithium such as SiO_(β)(0<β<2), SnO₂, vanadiumoxide, and lithium vanadium oxide; or a composite including the metalliccompound and the carbonaceous material such as a Si—C composite or aSn—C composite, and any one thereof or a mixture of two or more thereofmay be used. Also, a metallic lithium thin film may be used as thenegative electrode active material. Furthermore, both low crystallinecarbon and high crystalline carbon may be used as the carbon material.Typical examples of the low crystalline carbon may be soft carbon andhard carbon, and typical examples of the high crystalline carbon may beirregular, planar, flaky, spherical, or fibrous natural graphite orartificial graphite, Kish graphite, pyrolytic carbon, mesophasepitch-based carbon fibers, meso-carbon microbeads, mesophase pitches,and high-temperature sintered carbon such as petroleum or coal tar pitchderived cokes.

Also, the binder and the conductive agent may be the same as thosepreviously described in the positive electrode.

In the lithium secondary battery, the separator separates the negativeelectrode and the positive electrode and provides a movement path oflithium ions, wherein any separator may be used as the separator withoutparticular limitation as long as it is typically used in a lithiumsecondary battery, and particularly, a separator having highmoisture-retention ability for an electrolyte as well as low resistanceto the transfer of electrolyte ions may be used. Specifically, a porouspolymer film, for example, a porous polymer film prepared from apolyolefin-based polymer, such as an ethylene homopolymer, a propylenehomopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer,and an ethylene/methacrylate copolymer, or a laminated structure havingtwo or more layers thereof may be used. Also, a typical porous nonwovenfabric, for example, a nonwoven fabric formed of high melting pointglass fibers or polyethylene terephthalate fibers may be used.Furthermore, a coated separator including a ceramic component or apolymer material may be used to secure heat resistance or mechanicalstrength, and the separator having a single layer or multilayerstructure may be selectively used.

Also, the electrolyte used in the present invention may include anorganic liquid electrolyte, an inorganic liquid electrolyte, a solidpolymer electrolyte, a gel-type polymer electrolyte, a solid inorganicelectrolyte, or a molten-type inorganic electrolyte which may be used inthe preparation of the lithium secondary battery, but the presentinvention is not limited thereto.

Specifically, the electrolyte may include an organic solvent and alithium salt.

Any organic solvent may be used as the organic solvent withoutparticular limitation so long as it may function as a medium throughwhich ions involved in an electrochemical reaction of the battery maymove. Specifically, an ester-based solvent such as methyl acetate, ethylacetate, y-butyrolactone, and s-caprolactone; an ether-based solventsuch as dibutyl ether or tetrahydrofuran; a ketone-based solvent such ascyclohexanone; an aromatic hydrocarbon-based solvent such as benzene andfluorobenzene; or a carbonate-based solvent such as dimethyl carbonate(DMC), diethyl carbonate (DEC), methylethyl carbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC);an alcohol-based solvent such as ethyl alcohol and isopropyl alcohol;nitriles such as R-CN (where R is a linear, branched, or cyclic C2-C20hydrocarbon group and may include a double-bond aromatic ring or etherbond); amides such as dimethylformamide; dioxolanes such as1,3-dioxolane; or sulfolanes may be used as the organic solvent. Amongthese solvents, the carbonate-based solvent may be used, and, forexample, a mixture of a cyclic carbonate (e.g., ethylene carbonate orpropylene carbonate) having high ionic conductivity and high dielectricconstant, which may increase charge/discharge performance of thebattery, and a low-viscosity linear carbonate-based compound (e.g.,ethylmethyl carbonate, dimethyl carbonate, or diethyl carbonate) may beused. In this case, the performance of the electrolyte solution may beexcellent when the cyclic carbonate and the chain carbonate are mixed ina volume ratio of about 1:1 to about 1:9.

The lithium salt may be used without particular limitation as long as itis a compound capable of providing lithium ions used in the lithiumsecondary battery. Specifically, LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiSbF₆,LiAlO₄, LiAlCl₄, LiCF₃SO₃, LiC₄F₉SO₃, LiN(C₂F₅SO₃)₂, LiN(C₂F₅SO₂)₂,LiN(CF₃SO₂)₂, LiCl, LiI, or LiB(C₂O₄)₂ may be used as the lithium salt.The lithium salt may be used in a concentration range of 0.1 M to 2.0 M.In a case in which the concentration of the lithium salt is includedwithin the above range, since the electrolyte may have appropriateconductivity and viscosity, excellent performance of the electrolyte maybe obtained and lithium ions may effectively move.

In order to improve lifetime characteristics of the battery, suppressthe reduction in battery capacity, and improve discharge capacity of thebattery, at least one additive, for example, a halo-alkylenecarbonate-based compound such as difluoroethylene carbonate, pyridine,triethylphosphite, triethanolamine, cyclic ether, ethylenediamine,n-glyme, hexaphosphoric triamide, a nitrobenzene derivative, sulfur, aquinone imine dye, N-substituted oxazolidinone, N,N-substitutedimidazolidine, ethylene glycol dialkyl ether, an ammonium salt, pyrrole,2-methoxy ethanol, or aluminum trichloride, may be further added to theelectrolyte in addition to the electrolyte components. In this case, theadditive may be included in an amount of 0.1 wt % to 5 wt % based on atotal weight of the electrolyte.

As described above, since the lithium secondary battery including thepositive electrode active material according to the present inventionstably exhibits excellent discharge capacity, output characteristics,and capacity retention, the lithium secondary battery is suitable forportable devices, such as mobile phones, notebook computers, and digitalcameras, and electric cars such as hybrid electric vehicles (HEVs).

Thus, according to another embodiment of the present invention, abattery module including the lithium secondary battery as a unit celland a battery pack including the battery module are provided.

The battery module or the battery pack may be used as a power source ofat least one medium and large sized device of a power tool; electriccars including an electric vehicle (EV), a hybrid electric vehicle, anda plug-in hybrid electric vehicle (PHEV); or a power storage system.

Hereinafter, examples of the present invention will be described indetail in such a manner that it may easily be carried out by a personwith ordinary skill in the art to which the present invention pertains.The invention may, however, be embodied in many different forms andshould not be construed as being limited to the examples set forthherein.

EXAMPLE 1

In a 5L batch-type reactor set at 60° C., NiSO₄, CoSO₄, and MnSO₄ weremixed in water in amounts such that a molar ratio ofnickel:cobalt:manganese was 50:20:30 to prepare a precursor-formingsolution with a concentration of 2.4 M.

1 L of deionized water was put in a co-precipitation reactor (capacity 5L), the reactor was then purged with nitrogen gas at a rate of 2 L/minto remove dissolved oxygen in the water and create a non-oxidizingatmosphere in the reactor. Thereafter, 10 ml of a 25% NaOH aqueoussolution was added, and stirring was then performed at a speed of 1,200rpm and a temperature of 60° C. to maintain a pH at 12.0.

Subsequently, a co-precipitation reaction was performed for 18 hourswhile adding the precursor-forming solution at a rate of 180 ml/hrtogether with a NaOH aqueous solution and a NH₄OH aqueous solution toform particles of a nickel-cobalt-manganese-containing hydroxide(Ni_(0.50)Co_(0.20)Mn_(0.30)(OH)₂). The hydroxide particles wereseparated, washed, and then dried in an oven at 120° C. to prepare apositive electrode active material precursor. The positive electrodeactive material precursor thus prepared was in the form of a secondaryparticle in which primary particles were aggregated.

The positive electrode active material precursor thus prepared and alithium raw material, LiOH, were added to a Henschel mixer (700 L) suchthat a final molar ratio of Li/M(Ni, Co, Mn) was 1.02, and were mixed ata center speed of 300 rpm for 20 minutes. The mixed powder was put in analumina crucible with a size of 330 mm×330 mm and primarily sintered at1,010° C. for 10 hours under an oxygen (O₂) atmosphere to form a primarysintered product.

Thereafter, the primary sintered product was ground at a feedingpressure of 80 psi and a grinding pressure of 60 psi using a jet mill.

The ground primary sintered product was put in an alumina crucible witha size of 330 mm×330 mm and secondarily sintered at 850° C. for 5 hoursunder an oxygen (O₂) atmosphere to prepare a positive electrode activematerial.

EXAMPLE 2

A positive electrode active material was prepared in the same manner asin Example 1 except that the primary sintering was performed at 980° C.for 10 hours.

EXAMPLE 3

A positive electrode active material was prepared in the same manner asin Example 1 except that the secondary sintering was performed at 800°C. for 5 hours.

EXAMPLE 4

A positive electrode active material was prepared in the same manner asin Example 1 except that a precursor was prepared such that a molarratio of nickel:cobalt:manganese was 80:10:10, the primary sintering wasperformed at 930° C. for 10 hours, and the secondary sintering wasperformed at 750° C. for 5 hours.

EXAMPLE 5

A positive electrode active material was prepared in the same manner asin Example 4 except that the primary sintering was performed at 900° C.for 10 hours.

EXAMPLE 6

A positive electrode active material was prepared in the same manner asin Example 4 except that the secondary sintering was performed at 700°C. for 5 hours.

Comparative Example 1

A positive electrode active material was prepared in the same manner asin Example 1 except that the secondary sintering was not performed.

Comparative Example 2

A positive electrode active material was prepared in the same manner asin Example 1 except that the primary sintering was performed at 910° C.for 20 hours, and the secondary sintering was not performed.

Comparative Example 3

A positive electrode active material was prepared in the same manner asin Example 4 except that the secondary sintering was not performed.

Comparative Example 4

A positive electrode active material was prepared in the same manner asin Example 4 except that the primary sintering was performed at 810° C.for 20 hours, and the secondary sintering was not performed.

Experimental Example 1: Positive Electrode Active Material Observation

Images of the positive electrode active materials prepared in Examples 1to 6 and Comparative Examples 1 to 4, which were magnified with ascanning electron microscope (SEM), are illustrated in FIGS. 1 to 10,and images of the positive electrode active materials observed with atransmission electron microscope (TEM) are illustrated in FIGS. 11 to14.

Referring to FIGS. 1 to 7 and 9, the positive electrode active materialsprepared in Examples 1 to 6 of the present invention and ComparativeExamples 1 and 3 formed a primary structure of a single particle, but,referring to FIGS. 8 and 10, it may be confirmed that the positiveelectrode active materials prepared in Comparative Examples 2 and 4 werein the form of a secondary particle in which primary particles wereaggregated to each other.

Referring to FIGS. 11 to 14, with respect to the positive electrodeactive materials prepared in Examples 1 and 4 of the present invention,it may be confirmed that a resistance portion having a cubic rock-saltstructure of space group Fm-3m was formed on the surface of a coreportion having a layered crystal structure of space group R-3m, and,with respect to the positive electrode active materials prepared inComparative Examples 1 and 3 in which the secondary sintering was notperformed, a layered crystal structure of space group R-3m wasidentified in both core portion and surface.

Experimental Example 2: Crystallite Size

Crystallite sizes of the positive electrode active materials prepared inExamples 1 and 2 and Comparative Examples 1 and 2 were measured. Thecrystallite sizes were measured by XRD (Ultima IV) and their values werecalculated.

TABLE 1 Crystallite size (nm) Example 1 200 Example 2 180 Example 3 205Example 4 200 Example 5 185 Example 6 190 Comparative Example 1 205Comparative Example 2 165 Comparative Example 3 190 Comparative Example4 150

Referring to Table 1, the positive electrode active materials ofExamples 1 to 6 and Comparative Examples 1 and 3, in whichover-sintering was performed in the primary sintering, had a crystallitesize of 180 nm or more. However, the positive electrode active materialsof Comparative Examples 2 and 4, in which the primary sinteringtemperature was low, had a crystallite size of less than 180 nm.

Experimental Example 3: Thermal Stability Evaluation

In order to evaluate thermal stabilities of the positive electrodeactive materials prepared in Examples 1 to 6 and Comparative Examples 1to 4, a heat flow according to the temperature was measured using adifferential scanning calorimeter (SENSYS Evo by SETARAMInstrumentation).

Specifically, each of the positive electrode active materials preparedin Examples 1 to 6 and Comparative Examples 1 to 4, a carbon blackconductive agent, and a PVdF binder were mixed in an N-methylpyrrolidonesolvent at a weight ratio of 96:2:2 to prepare a positive electrodematerial mixture, and one surface of an aluminum current collector wascoated with the positive electrode material mixture, dried at 100° C.,and then rolled to prepare a positive electrode.

Lithium metal was used as a negative electrode.

Each lithium secondary battery was prepared by preparing an electrodeassembly by disposing a porous polyethylene separator between thepositive electrode and negative electrode prepared as described above,disposing the electrode assembly in a case, and then injecting anelectrolyte solution into the case. In this case, the electrolytesolution was prepared by dissolving 1.0 M lithium hexafluorophosphate(LiPF₆) in an organic solvent composed of ethylene carbonate/ethylmethylcarbonate/diethyl carbonate (mixing volume ratio of EC/EMC/DEC=3/4/3).

Each lithium secondary battery half cell thus prepared was disassembledin a charged state, a state of charge (SOC) of 100%, to introduce thepositive electrode and a new electrolyte solution into a cell for DSCmeasurement, and measurement was performed while increasing thetemperature from room temperature to 400° C. at a rate of 10° C. perminute. The results thereof are presented in Table 2.

TABLE 2 DSC main peak Maximum heat flow (° C.) (W/g) Example 1 291 4Example 2 284 6 Example 3 281 6 Example 4 226 10 Example 5 213 12Example 6 221 10 Comparative Example 1 280 10 Comparative Example 2 27023 Comparative Example 3 215 30 Comparative Example 4 210 70

Referring to Table 2, with respect to Examples 1 to 3 of the presentinvention, main peaks with a maximum heat flow appeared at relativelyhigher temperatures than those of Comparative Examples 1 and 2, and itmay be confirmed that the maximum heat flows were significantly reduced.Also, with respect to Examples 4 to 6, main peaks with a maximum heatflow appeared at relatively higher temperatures than those ofComparative Examples 3 and 4, and it may be confirmed that the maximumheat flows were significantly reduced. Accordingly, it may be understoodthat thermal stabilities of Examples 1 to 6 were significantly improvedin comparison to those of Comparative Examples 1 to 4.

Since the positive electrode active material prepared in ComparativeExample 2, which had a secondary particle shape due to a difference inprimary sintering conditions greatly influencing a single particle shapeand Brunauer-Emmett-Teller (BET), had high reactivity, it may beconfirmed that the temperature of the main peak was lower than that ofComparative Example 1 and the maximum heat flow was high. With respectto Comparative Example 1, since its primary sintering conditions weresimilar to those of Examples 1 to 3, its single particle shape and BETwere controlled to levels similar to those of Examples 1 to 3, but itmay be expected that a low main peak temperature and a high maximum heatflow were exhibited due to the absence of the surface Fm-3m resistanceportion by the secondary sintering. Similarly, with respect toComparative Examples 3 and 4, it may be confirmed that, with respect toComparative Example 4, the main peak temperature was lower and themaximum heat flow was higher than that of Comparative Example 3 due tochanges in single particle shape and BET caused by the difference inprimary sintering conditions. With respect to Comparative Example 3,since its primary sintering conditions were similar to those of Examples4 to 6, its single particle shape and BET were similarly controlled, butit may be expected that a lower main peak temperature and a highermaximum heat flow than those of Examples 4 to 6 were exhibited due tothe absence of the surface Fm-3m resistance portion by the secondarysintering.

Experimental Example 4: Life Characteristics Evaluation

Each of the lithium secondary battery half cells prepared as inExperimental Example 3 by using each of the positive electrode activematerials prepared in Examples 1 to 6 and Comparative Examples 1 to 4was charged at 0.2 C to a voltage of 4.4 V (Examples 1 to 3, ComparativeExamples 1 and 2) or 4.25 V (Examples 4 to 6, Comparative Examples 3 and4) in a constant current/constant voltage (CCCV) mode at 25° C.(termination current of 1/20 C), and discharged at a constant current of0.2 C to a voltage of 3.0 V to measure initial charge and initialdischarge capacities. Thereafter, each half cell was charged at 0.7 C toa voltage of 4.4 V (Examples 1 to 3, Comparative Examples 1 and 2) or4.25 V (Examples 4 to 6, Comparative Examples 3 and 4) in a CCCV mode,and discharged at a constant current of 0.5 C to a voltage of 3.0 V tomeasure capacity retention when 30 cycles of charge and discharge wereperformed and thus, life characteristics were evaluated. The resultsthereof are presented in Table 3.

TABLE 3 Initial discharge capacity Capacity retention (mAh/g) (%) (@30^(th) cycle) Example 1 176 90 Example 2 176 90 Example 3 175 89Example 4 195 81 Example 5 194 80 Example 6 195 82 Comparative Example 1178 87 Comparative Example 2 180 83 Comparative Example 3 195 74Comparative Example 4 200 69

Referring to Table 3, with respect to the positive electrode activematerials prepared in Examples 1 to 3, initial capacities wereequivalent or somewhat inferior to those of Comparative Examples 1 and2, but it may be confirmed that life characteristics were significantlyimproved. Also, with respect to the positive electrode active materialsprepared in Examples 4 to 6, initial capacities were equivalent orsomewhat inferior to those of Comparative Examples 3 and 4, but it maybe confirmed that life characteristics were significantly improved.

Since the positive electrode active material prepared in ComparativeExample 2, which had a secondary particle shape due to the difference inprimary sintering conditions greatly influencing a single particle shapeand BET, had large BET, resistance was low due to the large surfacearea, and thus, it may be confirmed that it exhibited higher dischargecapacity than Comparative Example 1 and cycle characteristics werepoorer due to a large reaction area. With respect to Comparative Example1, since its primary sintering conditions were similar to those ofExamples 1 to 3, its single particle shape and BET were controlled tolevels similar to those of Examples 1 to 3, but, since resistance waslow due to the absence of the surface Fm-3m resistance portion by thesecondary sintering, high discharge capacity was exhibited and surfacereactivity was relatively increased, and thus, it may be expected thatpoor cycle retention characteristics were exhibited. Similarly, withrespect to Comparative Examples 3 and 4, it may be confirmed that, withrespect to Comparative Example 4, discharge capacity was higher andcycle characteristics were poorer than those of Comparative Example 3due to the changes in single particle shape and BET caused by thedifference in primary sintering conditions. With respect to ComparativeExample 3, since its primary sintering conditions were similar to thoseof Examples 4 to 6, its single particle shape and BET were similarlycontrolled, but it may be expected that higher discharge capacity andhigher surface reactivity than those of Examples 4 to 6 were exhibiteddue to the absence of the surface Fm-3m resistance portion by thesecondary sintering.

1. A method of preparing a positive electrode active material for asecondary battery, comprising: preparing a positive electrode activematerial precursor including nickel (Ni), cobalt (Co), and manganese(Mn) in which an amount of the nickel (Ni) in a total amount of metalsis less than 60 mol %; mixing the positive electrode active materialprecursor and a lithium raw material, and performing primary sinteringon the mixture at a primary sintering temperature of 980° C. or more toform a primary sintered product; and performing secondary sintering onthe primary sintered product at a secondary sintering temperature of900° C. or less to form a lithium composite transition metal oxide,wherein a particle of the lithium composite transition metal oxide iscomposed of a single particle and the particle of the lithium compositetransition metal oxide comprises a core portion having a layered crystalstructure of space group R-3m; and a resistance portion which is formedon a surface of the core portion and has a cubic rock-salt structure ofspace group Fm-3m.
 2. The method of claim 1, wherein the primarysintering temperature is in a range of 990° C. to 1,050° C.
 3. Themethod of claim 1, wherein the secondary sintering temperature is in arange of 600° C. to 900° C.
 4. A method of preparing a positiveelectrode active material for a secondary battery, comprising: preparinga positive electrode active material precursor including nickel (Ni),cobalt (Co), and manganese (Mn) in which an amount of the nickel (Ni) ina total amount of metals is 60 mol % or more; mixing the positiveelectrode active material precursor and a lithium raw material, andperforming primary sintering on the mixture at a primary sinteringtemperature of 850° C. or more to form a primary sintered product; andperforming secondary sintering on the primary sintered product at asecondary sintering temperature of 800° C. or less to form a lithiumcomposite transition metal oxide, wherein a particle of the lithiumcomposite transition metal oxide is composed of a single particle andthe particle of the lithium composite transition metal oxide comprises acore portion having a layered crystal structure of space group R-3m; anda resistance portion which is formed on a surface of the core portionand has a cubic rock-salt structure of space group Fm-3m.
 5. The methodof claim 4, wherein the primary sintering temperature is in a range of850° C. to 1,000° C.
 6. The method of claim 4, wherein the secondarysintering temperature is in a range of 500° C. to 800° C.
 7. The methodof claim 1, further comprising milling the primary sintered productafter the primary sintering and before the secondary sintering.
 8. Themethod of claim 1, wherein the positive electrode active in materialprecursor is a secondary particle in which primary particles areaggregated.
 9. The method of claim 1, wherein the secondary sintering isperformed such that the lithium composite transition metal oxideparticle is composed of a primary particle having an average particlediameter (D50) of 1 μm to 20 μm.
 10. The method of claim 1, wherein thesecondary sintering is performed such that the core portion has acrystallite size of 180 nm to 400 nm.
 11. A positive electrode activematerial for a secondary battery, the positive electrode active materialcomprising a lithium composite transition metal oxide including nickel(Ni), cobalt (Co), and manganese (Mn), wherein a particle of the lithiumcomposite transition metal oxide comprises a core portion and aresistance portion formed on a surface of the core portion, and iscomposed of a single particle, wherein the core portion has a layeredcrystal structure of space group R-3m, and the resistance portion has acubic rock-salt structure of space group Fm-3m.
 12. The positiveelectrode active material for a secondary battery of claim 11, whereinthe lithium composite transition metal oxide particle is composed of aprimary particle having an average particle diameter (D50) of 1 μm to 20μm.
 13. The positive electrode active material for a secondary batteryof claim 11, wherein the resistance portion is formed on a part orentirety of the surface of the core portion.
 14. The positive electrodeactive material for a secondary battery of claim 11, wherein the coreportion has a crystallite size of 180 nm to 400 nm.
 15. The positiveelectrode active material for a secondary battery of claim 11, whereinan amount of the nickel (Ni) in a total amount of metals excludinglithium (Li) is less than 60 mol %, a main peak with a maximum heat flowis measured at 280° C. or more, and the maximum heat flow is 8 W/g orless when the positive electrode active material is thermally analyzedby differential scanning calorimetry (DSC).
 16. The positive electrodeactive material for a secondary battery of claim 11, wherein an amountof the nickel (Ni) in a total amount of metals excluding lithium (Li) is60 mol % or more, a main peak with a maximum heat flow is measured at210° C. or more, and the maximum heat flow is 15 W/g or less when thepositive electrode active material is thermally analyzed by differentialscanning calorimetry (DSC).
 17. A positive electrode for a secondarybattery, the positive electrode comprising the positive electrode activematerial of claim
 11. 18. A lithium secondary battery comprising thepositive electrode of claim
 17. 19. The positive electrode activematerial for a secondary battery of claim 11, wherein an amount of thenickel (Ni) in a total amount of metals excluding lithium (Li) is lessthan 60 mol %, a main peak with a maximum heat flow is measured at 280°C. to 300° C., and the maximum heat flow is 8 W/g or less when thepositive electrode active material is thermally analyzed by differentialscanning calorimetry (DSC).
 20. The positive electrode active materialfor a secondary battery of claim 11, wherein an amount of the nickel(Ni) in a total amount of metals excluding lithium (Li) is 60 mol % ormore, a main peak with a maximum heat flow is measured at 210° C. to250° C., and the maximum heat flow is 15 W/g or less when the positiveelectrode active material is thermally analyzed by differential scanningcalorimetry (DSC).